The assembly of motor vehicles of the type that have been designed on the basis of modular construction principles allows the module containing the engine, i.e., the engine module, to be inserted into the vehicle body vertically from below or above, or horizontally from the front. In the case of prior art modular vehicle designs, the arrangement has been such that the heating system, consisting of the heat exchanger and the enclosure surrounding same, was being completely pre-assembled on one side of the fire wall, i.e., the side facing the occupant compartment, and the fire wall was part of the so-called cockpit module. Since the heat exchanger of the heating system is an integral component of the engine cooling water circuit, this prior art modular design concept does not provide for the ability to conduct, prior to final installation of the engine module into the vehicle body, engine test runs and other testing procedures for ascertaining proper operation of all engine functions, because such tests would require the heat exchanger to be connected to the engine cooling system. In other words, the arrangement wherein the heat exchanger of the heating system is mounted on the side of the fire wall facing the occupant compartment and is part of the cockpit module rather than part of the engine module, does not permit the engine cooling water system, prior to installation of the engine into the vehicle body, to be closed so that the necessary engine tests can be conducted.